This invention relates generally to thyristor maxtrix arrangements, and more particularly, to a thyristor matrix having at least four columns, each column having a plurality of thyristors connected in series, the series thyristors being connected to one another at junction points; selected ones of the junction points in different thyristor columns being connected to one another via cross-connections. The matrix conducts current in each of first and second conduction directions; equal numbers of thyristors being assigned to the conduction directions.
Columns of thyristors are known, for example, from U.S. Pat. No. 3,943,426. In this known arrangement, disc type thyristors are stacked upon each other and held resiliently together. A heat sink is inserted between two disc type thyristors and serves for conducting electric current. In order to handle large currents, several such columns can be connected in parallel. Moreover, in arrangements where conduction is desired in two directions, such thyristor columns can be arranged antiparallel. In one known arrangement, two columns of thyristors are connected in parallel with one another, and poled for conduction in a first direction, and two further columns of thyristors are also connected in parallel with one another, but are poled for conduction in the opposite direction. In this manner, a matrix having a total of four thyristor columns is produced. In order to ensure a uniform distribution of current in the known, commercially available, four-column thyristor equipment, all of the thyristors are connected to one another.
FIG. 1 shows a known thyristor matrix having four columns and two common terminals, A1 and A2. The known arrangement of FIG. 1 contains n thyristors in each column, the thyristors in the matrix being identified as T11 . . . , Tn1; T12 . . . , Tn2; T13 . . . , Tn3; and T14 . . . , Tn4. A plurality of heat sinks are each interconnected between two thyristors. The heat sinks are schematically indicated in FIG. 1, and designated with respective symbols K01 . . . , Kn4.
In this known arrangement, thyristor columns S1 and S2 are connected in parallel. Thyristor columns S3 and S4 are also connected in parallel with one another, but antiparallel to the combination of columns S1 and S2. Corresponding heat sinks K11 . . . , K14 are connected to one another, as are heat sinks K21 . . . , K24; and Kn1 . . . , Kn4. The heat sinks are connected to each other by means of cross-connections which are respectively identified as Q11 . . . , Qn3. Generally, the heat sinks identified as Kp1 . . . , Kp4, where p assumes the values one to n, are connected to each other. In this manner, the thyristors within the groups T11 . . . , T14 through Tn1 . . . , Tn4 are also connected to one another. All interrelated thyristors in the generalized group Tp1 . . . , Tp4, with their associated heat sinks Kp1 . . . , Kp4, of the four thyristor columns S1 to S4, shall be generally designated in the following discussion as level Ep of the thyristor arrangement. This known thyristor arrangement therefore comprises n levels E1 . . . , En.
Each of the levels E1 . . . , En of the thyristor arrangement is assigned one of common RC stages, R1, C1 . . . , Rn, Cn. The assigned RC stages are electrically disposed between the heat sinks of adjacent levels.
FIG. 2 is a schematic representation of the physical arrangement of the four thyristor columns S1 . . . , S4. The four thyristor columns are represented in this figure in a rectangular shape, the cross-connections being shown as leads. As a result of the physical configuration of the matrix, the distance between columns S1 and S3, or S2 and S4, is larger than the distance between adjacent columns. In addition, the cross-connections Q12 . . . , Qn2 are longer than the other cross-connections.
It is a problem with this known thyristor arrangement that the individual thyristors of the four columns, S1 . . . , S4, must be matched to each other in accordance with their respective dynamic forward characteristics. FIG. 3 shows a typical dynamic forward characteristic for heavy duty thyristors. As shown in FIG. 3, the curve of the thyristor current i.sub.t is a function of the anode-cathode voltage of the thyristor. Here, the firing voltage is designated as V.sub.Z, and the forward voltage for the thyristor current, i.sub.D, is designated with V.sub.D. Such a characteristic may result in failure of the thyristors to fire simultaneously, because one of the parallel connected thyristors may have a lower firing voltage than the other thyristors. The early firing thyristor would prevent the remaining thyristors from firing, and thereby conduct the entire current so as to be overloaded. Thus, in order to ensure substantially simultaneous firing of all thyristors, they must be sorted and matched with respect to their firing voltages V.sub.Z. In addition, the thyristors must also be sorted and matched with respect to their forward voltage V.sub.D because otherwise the parallel-connected thyristors would conduct unequal amounts of current. The requirement of sorting and matching thyristors with respect to two criteria presents major practical difficulties. Moreover, through aging or differences in the firing delay times, it may occur that only one of the thyristors fires while the remaining parallel-connected thyristors are below their firing voltages. In order to reliably exclude this possibility, transistors would have to be used for the parallel connections, because the firing voltages are lower than the forward voltages. The selection process, however, is quite expensive.
It is a further problem with the known thyristor arrangement that an uneven current distribution caused by a deviation in the characteristics of a thyristor is continued over the entire column. Although the connections between the thyristors through the heat sinks have very small inductances because of the small lengths of the connections, the longer cross-connections have more inductance so that equalization via the cross-connections does not occur, at least not over a short period of time, and the current distribution becomes increasingly worse.
It is known from BBC Silicon Converter Handbook, 1971, pages 95 and 96, that the current distribution among parallel-connected thyristors is improved by connecting an air-core choke in series with each individual thyristor of a parallel circuit. The effect of the air choke is that the thyristor which fires first will not take over the entire current immediately, and the voltage at the remaining thyristors increases because of the current change in the choke, so that the firing of the remaining thyristors is aided. However, such chokes for each individual thyristor not only increase the cost of the arrangement, but also the length because the chokes must be inserted between the thyristors and the heat sinks.
It is, therefore, an object of this invention to provide a thyristor arrangement wherein equalization of the firing times of, and the current distribution among, parallel-connected thyristors is achieved without the use of separate chokes.